The present invention relates to a microthermistor based fingerprint sensor.
Finger tips of human beings exhibit a pattern of ridges and valleys called a fingerprint. This structure is unique to every human being and has long been used for the identification of a person. Of particular interest are the ridge branching and ending points of the structure. These fingerprint features are called minutiae. The position and orientation of the minutiae can be used to characterise a fingerprint and thus minimise the information that has to be stored and processed for identification purposes, while still keeping an accurate fingerprint representation.
Fingerprint identification can be a time consuming activity when performed by human operators. This explains why automation of fingerprint identification has received considerable attention. Automated fingerprint identification requires the implementation of two main functions. First, the fingerprint pattern of ridges and valleys must be translated into an electronic format representative of the fingerprint pattern. This can be achieved through the use of various sensor types and arrangements. The fingerprint electronic representation can then be stored in a memory, if convenient, for further analysis. Second, the electronic data must be processed in order to achieve recognition. Various algorithms have been used for this purpose in the past. Some of them are based on a 2-dimensional representation of the fingerprint while others are based on a 1-dimensional fingerprint representation (see, for example, U.S. Pat. No. 5,745,046). Often, these algorithms make use of minutiae to characterise a fingerprint.
Advances in microelectronics and micro-machining have allowed substantial miniaturisation of fingerprint identification systems, which open a wide application range for these devices. They can be integrated to cars and houses as a replacement for keys and locks. They can be used to control access to high security areas. Fingerprint identification systems can replace Personal Identification Numbers (PINs) and ATM cards. They could also be placed in devices like cellular phones, computers or firearms to prevent unauthorised use of the same. Finally, law enforcement forces can bring them in the field as a very powerful tool for real-time identification of a person.
Historically, the first medium used for fingerprint recording was ink on paper. This process does not provide a good resolution and is not very convenient for users, as it involves the use of ink on a finger, which is then transferred to a piece of paper. After having placed the fingerprint on the piece of paper, the user must clean off the ink, which makes the process messy. Moreover, using these records for fingerprint identification is a very cumbersome and time consuming process.
Many optical systems have been implemented for fingerprint recording (see, for example, U.S. Pat. Nos. 5,548,394, 5,937,557, 5,892,599 and 5,920,384). Generally, an image of the fingerprint is projected on a 2D detector array or is scanned on a 1D detector array. Optical arrangements used to produce this image are very diverse and can make use, for example, of optical fiber bundles (U.S. Pat. No. 5,937,557), holographic optical elements (U.S. Pat. No. 5,892,599) or mini-prism arrays (U.S. Pat. No. 5,920,384). Some approaches take advantage of Frustrated Total Internal Reflection (FTIR) to enhance the contrast between the fingerprint ridges and valleys (U.S. Pat. No. 5,548,394). In these systems, the finger to be identified is typically pressed against a prism which provides the required FTIR. One operational drawback of this approach is that the optical surfaces in contact with the finger must be cleaned regularly to maintain the system performance. Moreover, such systems are typically large and the alignment of the various optical components part of the system must be kept within tight tolerances.
Other fingerprint identification systems sense the distribution of some electrical properties representative of the finger skin pattern. In some cases, the sensor is a micro-switch array (U.S. Pat. No. 3,781,855). The skin being an electrical conductor, direct contact of the fingerprint ridges with the micro-switches closes a circuit. Micro-switches of the array beneath fingerprint valleys remain opened. The pattern of closed and open switches in the array provides a representation of the fingerprint structure. Reading of this pattern is achieved through an integrated circuit, which is part of the micro-switch array chip. A variation of this approach involves measurements of the skin electrical resistance instead of simple switch state reading. It is therefore possible to evaluate to which extent the finger is in contact with various points of the underlying chip. This allows a better resolution of the transition zone between ridges and valleys and the fingerprint representation obtained is more accurate.
Other systems measure the capacitance between the finger skin and microelectrodes, as described for example in U.S. Pat. No. 5,325,442. In this approach, the finger skin is one of the capacitor plates and the microelectrode is the other capacitor plate. The value of this capacitance is a function of the distance between the finger skin and the electrode. When the finger is placed on a microelectrode array, the capacitance variation pattern measured from electrode to electrode gives a mapping of the distance between the finger skin and the various microelectrodes underneath. This corresponds to the ridge and valley structure on the finger tip. Here again, the parameter of importance, i.e. the capacitance, is read using a integrated circuit fabricated on the same substrate as the microelectrode array. More recent systems involve measurement of the finger skin equivalent complex impedance as part of a read-out circuit (U.S. Pat. No. 5,953,441) instead of simply measuring the ohmic resistance or the capacitance as described above.
Finally, fingerprint recording devices, closely related to the capacitance measurement systems described above, make use of electric field sensor array in order to obtain the fingerprint representation (U.S. Pat. No. 5,940,526).
Another category of fingerprint reading systems relies on pressure sensor arrays. In this case, various mechanisms are used to measure the pressure applied by the finger tip at different points of the sensor array. For points where a fingerprint ridge is in contact with a sensor, the pressure is high, while it is null for sensors underneath fingerprint valleys. The sensing mechanisms used here are very diverse. Some systems are based on micro-switch arrays (as described in U.S. Pat. Nos. 4,577,345 and 5,400,662). The state of the individual micro-switches (on or off) depends on the amount of pressure applied on them. Implementation of this approach is often done using a thin membrane which is electrically conductive or has a conductive layer on its side facing the switch array. This membrane must be soft enough so it takes the fingerprint shape when pressed by the finger against the switch array. Membrane points corresponding to fingerprint ridges touch the sensor array and close the underlying switches. For the membrane points corresponding to fingerprint valleys, the micro-switches remain in the off state. The main disadvantage of this approach is that it is very demanding on the membrane properties, as it must be conductive (at least partially), very soft and yet capable of withstanding repeatedly the deformations induced by a user""s finger. An improved version of this system, as described in U.S. Pat. No. 5,844,287, makes use of an array of micro-membranes instead of a single membrane. One micro-membrane is fabricated over each micro-switch in the array. When a fingerprint ridge touches a micro-membrane, it brings it into contact with the underlying circuit which closes the associated micro-switch. This approach is less demanding on the membrane material, as the micro-membrane deformation can be made smaller.
For the fingerprint recording systems based on pressure sensor arrays described above, the transition zone between ridges and valleys is very sharp as the sensor signal is binary. This can cause some inaccuracy on the recorded fingerprint feature size and position. Other pressure sensor types can be used to avoid such problems. For example, pressure sensors based on capacitance readings can achieve grey levels in fingerprint representations (see U.S. Pat. Nos. 4,353,056 and 5,844,287).
These sensors typically include a compressible dielectric layer sandwiched between two electrodes. When pressure is applied to the top electrode, the inter-electrode distance changes, which modifies the capacitance associated with this structure. The higher the pressure applied, the larger the sensor capacitance gets. Arrays of such sensors combined with a read-out integrated circuit can be used for fingerprint acquisition. When a finger is brought into contact with such an array, fingerprint ridges correspond to the highest pressure point, while no pressure is applied at points associated with the fingerprint valleys. A whole range of intermediate pressures can be read for the transition zone between fingerprint ridge and valleys. This feature allows a grey level fingerprint representation which is more accurate than simple black and white recording. If the compressible dielectric material mentioned above is replaced by a compressible material exhibiting a relatively good electrical conductivity, the system concept described above can still be used but here, the sensor electrical resistance is read instead of the sensor capacitance (this solution is investigated in U.S. Pat. No. 5,745,046). This last system design still allows the grey level representation of fingerprints but has the disadvantages of consuming more energy.
Other systems take advantage of the fact that when a finger is pressed against a surface, the pressure is at a maximum at points corresponding to fingerprint ridges, null at points associated with fingerprint valleys and has intermediate values in the transition zone between ridges and valleys. In some cases, arrays of sensors made of piezoelectric material are used (see U.S. Pat. No. 4,394,773 and French application no. FR 2736179-A1). These sensors produce a voltage which is function of the pressure applied on them. The voltage spatial distribution in the sensor array gives a grey level fingerprint representation. In other systems, each sensing cell comprises a transistor whose operating point is a function of the pressure applied to the cell. The transistor operating point is affected either by direct influence of the pressure on the transistor or through modification of the properties of an electrical component sensitive to pressure and part of the circuit establishing the transistor operating point. The distribution pattern of the transistor operating point through the sensing cell array provides a mapping of the pressure field generated by the finger on the chip. As was already mentioned above, this pressure field gives an accurate representation of the fingerprint.
The spatial distribution of the thermal conductivity between a finger tip and a substrate against which it is pressed has also been used to obtain fingerprint representation in electronic format (see international application no. WO 96132061). For this type of system, a sensor array is heated and the heat exchange between the finger and the underlying sensors is monitored through a sensor temperature variation measurement. A large change in the sensor temperature indicates a large heat exchange between the considered sensor and the finger at this point. Large heat exchanges, in turn, correspond to high thermal conductivity points for which the thermal contact between the finger and the sensor is very good. Following this approach, the highest thermal conductivity points map the fingerprint ridges structure, the lowest thermal conductivity points give the fingerprint valleys pattern and intermediate thermal conductivity points correspond to the transition zone between ridges and valleys.
As for most of the systems described above, the data reading from the sensor array is performed by an integrated circuit part of the substrate on which the sensor array is fabricated. The main drawback of using the thermal conductivity as the parameter for fingerprint recording is that the sensors must be heated and, ideally, this heating should be done separately for each individual sensor.
Finally, one category of fingerprint reading systems is based on temperature sensor arrays. In this particular case, the equilibrium temperature of each sensor is function of the thermal contact between the finger and the sensor. A good thermal contact, corresponding to fingerprint ridges, will typically induce a larger sensor temperature change than a bad thermal contact. The sensors for which the temperature remains unaffected by the finger contact, at least for some amount of time, correspond to fingerprint valleys. The system implementations reported include sensor made of pyroelectric material (EP 0840250-A1). In this case, sensor temperature change generates an electric charge within the sensor. The charge pattern within the array can be read using the appropriate circuit, similar to the one used in CCD cameras. As the charge generated within a sensor depends on the temperature change experienced by this sensor, a representation of the temperature field on the sensor array is obtained. This temperature field is directly related to the fingerprint structure as was mentioned above. The problem with this approach is that the charge pattern representing the fingerprint will quickly fade away if the temperature change is not regularly refreshed. This is why this particular system requires the sensors to be pulse-heated repeatedly, which constantly recreates the temperature transient necessary for the system operation.
Another approach to avoid this problem consists in sweeping the fingerprint across a array of such pyroelectric sensors which generates temperature transients as well. However, this method requires more elaborated data processing algorithms to retrieve the fingerprint representation. Moreover, it is much more demanding on the sensor response time and on the system data acquisition rate. In other systems based on temperature sensor arrays, each sensing cell comprises a transistor whose operating point is function of the cell temperature (U.S. Pat. No. 4,429,413). The transistor operating point is affected either by direct influence of the transistor temperature or through modification of the properties of an electrical component sensitive to temperature and part of the circuit establishing the transistor operating point. The distribution pattern of the transistor operating point through the sensing cell array provides a mapping of the temperature field generated by the finger on the chip. As was already mentioned above, this temperature field gives an accurate representation of the fingerprint.
A whole range of different approaches to fingerprint recording has been described in the open literature. The classical approach based on ink and paper has proven to be very unpractical and time consuming. Other systems based on optical imaging can achieved good performances but they are typically large and require regular maintenance. A broad family of fingerprint recording devices makes use of a sensor array combined with a read out integrated circuit, both components being fabricated on the same substrate. The parameters measured by the sensors vary greatly. For example, various electrical properties characterising the finger skin pattern have been used as measuring parameters in different fingerprint sensing systems. Ohmic resistance, capacitance, complex impedance and electric fields have all been mentioned as possible parameters in the description of such systems. Other systems are based on pressure sensors (micro-switches, piezoelectric sensors, etc.) These pressure sensor based systems often include membranes or micro-membranes that must withstand repeated deformation which can reduce the device lifetime. An Implementation of a fingerprint recording system making use of thermal conductivity sensors has also been described. In this particular system, the sensors must be heated and the heat exchange between individual sensors and the finger skin is monitored through sensor temperature variation measurements. The drawback of this method is the required sensor heating and the associated power consumption. Finally, fingerprint acquisition devices are based on temperature sensor arrays. Some of those use pyroelectric sensors which require pulse heating in order to work properly. Such pulse heating being inconvenient, other systems rely on fingerprint sweeping across a pyroelectric sensor array which provides the temperature transients necessary for such sensors. The disadvantages of this solution are that it involves much more complex fingerprint retrieval algorithm and it is much more demanding on the sensor response time and on the system data acquisition rate. Other implementations of temperature sensitive fingerprint recording devices use transistors as temperature sensor. In this case, the operating point of the various transistors within the sensing cell array varies as a function of temperature either directly or through temperature sensitive electrical components part of the circuit establishing the transistor operating point.
The patents more closely related to the present invention involve temperature or thermal conductivity sensor arrays. In the approach based on a thermal conductivity sensor array, each sensor must be heated. Other patents describe systems based on temperature sensor array. Some of them use pyroelectric sensors which require temperature transients to operate properly.
It is an object of the present invention to prove a fingerprint sensor which is small; demonstrates high sensitivity, low power consumption, high data acquisition rate and is insensitive to vibration and pressure. The sensor of the present invention produces a high contrast fingerprint representation and is not subject to repeated deformation which could potentially reduces its lifetime. In accordance with the invention, these and other objects are achieved with a fingerprint sensor for transforming the ridge and valley pattern of a finger, hereinafter referred to as a fingerprint, into an electronic output signal, comprising:
(a) a microthermistor array for converting temperature variation into an electrical signal, said array being composed of a plurality of microthermistors, each of said thermistors being adapted to output an electrical signal proportional to a temperature variation;
(b) a read-out integrated circuit operatively connected to said microthermistor array for receiving said electrical signal and converting it into an electronic output signal representative of the ridge and valley structure of a finger; and
(c) a substrate for supporting said read-out integrated circuit and said microthermistor array.
The invention described in the present document is based on microthermistor array. The ohmic resistance of these sensors varies strongly with temperature. The parameter directly measured in this case is the temperature-induced resistance variation of each individual microthermistor. Signals from each of the microthermistor part of the array represent the fingerprint pattern.